Various types of color display technologies are known in the art, for example, projection display systems. In front projection displays, the projected images are viewed from a reflective viewing screen. In rear projection displays, the projected images are viewed through a transmissive viewing screen.
To produce color images, existing display devices use three primary colors, typically red, green and blue, collectively referred to as RGB. In sequential projection display systems, a plurality of color filter segments on a color wheel are sequentially introduced to the path of light of a polychromatic beam to sequentially produce three primary color light beams. The primary color light beams are modulated sequentially, for example, using a single Spatial Light Modulator (SLM) panel, and displayed on the reflective or transmissive viewing screen.
Such sequential RGB devices may have at least three transition regions (“spokes”), each formed between two adjacent color filter segments such that part of the polychromatic light beam passes through one color filter segment, and part of the light beam passes through an adjacent color filter segment. In such devices, the light passing through a spoke may produce a mixed color, e.g., a combination of the two primary colors produced by the adjacent filter segments. For example, a mixed red-green color may be produced in the spoke between the red and green filter segments. Displaying the mixed color may distort the color of the viewed image.
In order to avoid this effect, some systems prevent displaying the mixed colors, e.g., by switching the SLM to an “off” mode of operation during each transition between two adjacent color segments, and/or by adding a generally black or non-transparent segment between two successive filter segments. However, in many implementations, the “spot size” of the polychromatic light beam, i.e., the cross-sectional area of the light beam focused on the filter segments, may be relatively large, for example, the spot size may span about 15 degrees out of the 360 degrees span of the color wheel, whereby the spoke time between two adjacent filter segments may be significant compared to the display time of the primary color segments. Thus, switching the SLM to the “off” mode may substantially reduce the luminance of the display.
An important consideration in designing projection display devices is the display brightness. Thus, efforts are continually made to increase the optical efficiency of existing designs and, thereby, to increase the luminous output that can be obtained from a given light source.
In the display device described in U.S. Pat. No. 5,592,188 to Doherty et al., the SLM is not switched to the “off” mode during the spokes. Instead, the SLM is provided with “white image” data at the spokes, whereby a viewed white image is produced by a combination of the color components produced by all three spokes on the color wheel. This white image may be utilized to enhance white areas of the color image. Although a correct color balance may be maintained by this solution, the additional white light may reduce the color saturation of the displayed image.
U.S. Pat. No. 6,567,134 to Morgan et al. describes a spoke light recovery method. The method includes calculating from primary color data representing the color image special “secondary color” images, namely, yellow, cyan and magenta color components corresponding to three mixtures of primary color pairs. The method also includes providing the secondary color image to the SLM during the spoke time between the corresponding primary color segments.
The use of spoke light recovery in the devices described above requires manipulating the image input data in order to provide special signals, e.g. corresponding to the white/secondary color components.
In a more-than-three-primary sequential projection device, e.g., as described in International Application PCT/IL01/00527, entitled “Device, System and Method For Electronic True Color Display”, filed Jun. 7, 2001, and published Dec. 13, 2001 as WO 01/95544, assigned to the assignee of the present application, transition time becomes an important factor affecting display luminance, since the number of color filter segments is increased compared to RGB projection displays. Furthermore, the spokes of such multi-primary displays do not necessarily form a complementary set of colors.